1. Field of the Invention
This invention relates generally to light direction control, and more particularly, to techniques for the amplification of the deflection of an initially deflected light beam.
2. Description of the Related Art
Dynamic control of light beam propagation direction is a fundamental technique in optics. Direct applications include projection displays, entertainment, advertisement, laser printers, laser detection systems, laser scanning, optical communications, laser machining, etc.
Currently, there are four significant light deflection methods: electromechanical, acoustooptic, electrooptic, and electrically-controlled light grating methods. Of these four methods, electromechanical methods are used most frequently in commercial application products. Electromechanical methods use a rotating mirror or rotating prism as a mechanical device for changing light direction. These devices have a number of limitations due to the intrinsic nature of mechanical movement on a macroscopic scale. For example, they are relatively slow. Generally, it takes milliseconds to change the light beam from one direction to another direction. Also, such systems are susceptible to interference from mechanical vibration.
The other three light deflection methods contain no mechanical moving parts at macroscopic scale. However, the maximum deflection angle range is often a significant limiting factor to their performance. For example, the maximum deflection angle that the fully electronic control methods can provide is generally less than .+-.3.degree.. The small deflection angle essentially excludes electronic control methods from almost all important practical applications. Electromechanically-controlled rotating mirror devices can provide moderately larger deflection angle. The maximum deflection angle for two-dimensional electromechanically-controlled rotating mirrors is usually much less than .+-.30.degree., limited by the geometry of mechanical parts. And, in many important light scanning applications, such as laser radar systems, a much larger scanning angle range is often required. Thus, even the maximum deflection angle range of the electromechanical systems may still be insufficient.
In the prior art, light beam expanders are used for moderately increasing the deflection angle within a small angle range. There are two types of beam expander lens systems: Galileo-type and Kepler-type systems, in close analogy to the Galileo-type simple telescope and the Kepler-type simple telescope systems. A "simple telescope system" means that the telescope is built using only single-element lenses. A single-element lens can be a singlet, doublet, or triplet lens with conventional correction of aberrations. A single-element lens is different from a compound lens system, which is a combination of a number of lens elements. The Galileo-type beam expander uses a convex and a concave lens, and can only provide a small increase in the deflection angle, for example, less than a tripling of the initial deflection angle.
In the Kepler-type simple telescope systems, a beam expander is used in the reverse direction, increasing the deflection angle. The beam expander system uses two single-element lenses: a large lens and a small lens. For a collimated light beam with an incident angle .theta..sub.0, the following relationships hold true: tan .theta..sub.0 =(d/2)/f.sub.1 and tan .theta.=(d/2)/f.sub.2, where d is the diameter of the small lens, f.sub.1 is the focal length of the large lens, and f.sub.2 is the focal length of the small lens. Superficially, it might be deduced from the above relationships that tan .alpha./tan .alpha..sub.0 =f.sub.1 /f.sub.2, and it appears that as long as f1/f.sub.2 is sufficiently large, the deflection angle can be increased to a large value. In practice, however, this is not true. The above relationships depend on a practical restriction before the transformation is allowed. The maximum deflection angle allowed by the small single-element lens is rather small. Note that in prior art beam expanders, the small lens is only a single-element lens. And the aperture of a simple lens is generally rather limited. Only a small central area of the simple lens can only be utilized to ensure appropriate focusing quality. If the area of the lens to be used is too large, the focusing quality would be significantly degraded. The f-number for a single-element lens must be at least 2, that is, f.gtoreq.2d. Thus, the largest deflection angle value .theta. for the output light beam in the prior art beam expander is approximately .theta..sub.max.ltoreq.arctan [(d/2)/f]=arctan 0.25=14.degree.. And when the single-element lens f-number is 3, that is, f.gtoreq.3d, then .theta..sub.max.ltoreq.arctan 0.17=9.degree..
Note that actual maximum deflection angle value of the published data for acousto-optic deflectors with a beam expander is .theta..sub.max.ltoreq.10.degree..